This introduces a notion of synthetic atoms into `bv_decide`'s
reflection framework. An atom can be declared synthetic if its behavior
is fully specified by additional lemmas that are added in the process of
creating it. This is for example useful in the code that handles `if` as
the entire `if` block is abstracted as an atom and then two lemmas to
describe either branch are added. Previously this had the effect of
creating error messages about potentially unsound counterexamples, now
the synthetic atoms get filtered from the counter example generation.
